chore: use gapic-generator-python 1.3.1

Google APIs · copybara-github · commit 855b74d203de · 2022-09-07T11:31:33.000-07:00
PiperOrigin-RevId: 472772457
diff --git a/WORKSPACE b/WORKSPACE
@@ -278,7 +278,7 @@ load("@rules_python//python:pip.bzl", "pip_install")
 
 pip_install()
 
-_gapic_generator_python_version = "1.3.0"
+_gapic_generator_python_version = "1.3.1"
 
 http_archive(
     name = "gapic_generator_python",
